The present invention relates to a new and improved construction of an engineers wrench. Such wrench is a tool for tightening or loosening threaded joints and engages, during such process, a polygonal or multi-edged member of the threaded joint like, for example, the hexagonal or square head of a screw or a hexagonal nut. The wrench typically comprises a shaft and either at one or both of its ends a recess which is open on one side and defines the mouth of the wrench. The mouth is laterally delimited by two mutually oppositely disposed lateral surfaces at a spacing which defines a wrench width across the lateral surfaces. The wrench width is slightly wider than the diametral spacing between mutually opposed surfaces of the polygonal or multi-edged member of the threaded joint to be engaged by the wrench.
The two lateral surfaces of conventional wrenches like the associated surfaces of the multi-edged member to be engaged by the wrench, are mutually parallel flat surfaces. Due to the slight clearance existing between the lateral surfaces laterally delimiting the mouth opening of the wrench and the associated surfaces of the polygonal or multi-edged threaded joint member onto which the wrench is placed, the wrench will be tilted relative to the threaded joint member by a small amount during tightening or loosening of the threaded joint. As a result, the lateral surfaces of the wrench engage the edges of the threaded joint member, particularly the inner edge with respect to the mouth opening on one side and the outer edge with respect to the mouth opening on the opposite side. The torque for tightening or loosening the threaded joint, then, is transmitted via the edges.
During such operation, high surface pressures occur at the edges. Particularly upon repeated tightening and loosening of the threaded joint member, these high surface pressures will lead to wear at the respective edges whereby the function of the members will be negatively affected during the course of time.
The invention specifically relates to an engineers wrench which engages not the edges but the lateral surfaces of the polygonal or multi-edged member of the threaded joint.
Various constructions are known to realize such engagement.
German Patent No. 231,581 relates to a wrench in which the engagement surfaces of the jaws, i.e. the lateral surfaces delimiting the mouth opening, are formed by two flat surface portions which adjoin each other in a roof-like manner. Edges are formed between the surface portions and face each other. Outer surface portions of each one of the two lateral surfaces are disposed at a spacing, which corresponds to the wrench width, from inner surface portions of each one of the two mutually opposite lateral surfaces. Also in this wrench construction, the lateral surfaces delimiting the mouth opening press onto the edges of the polygonal or multi-edged member of the threaded joint during torque transmission.
German Patent No. 697,361 shows a wrench having a mouth opening which is delimited by flat surfaces and which is substantially adapted to the shape of a hexagonal nut. All of the corners of the mouth opening define cylindrical recesses. By virtue thereof, the wrench is intended to engage the nut only outside of the corners of the nut. Line contact of an edge will cause notching.
International Published Patent Applications Nos. 93/10945 and 94/23902 show a wrench in which the lateral surfaces delimiting the mouth opening initially, i.e. adjacent the outer edges which define the mouth opening, comprise respective mutually parallel flat surface portions. Adjoined thereto in the lateral surfaces, are disposed respective indentations which, in turn, are adjoined by convex-cylindrical sections. The convex-cylindrical sections, then, continuously merge with an arcuate mouth base. The radii of curvature of the convex-cylindrical sections are equal to half the wrench width. High surface pressures result also in this construction due to the small radii of curvature.
German Published Patent No. 1,088,437 shows a wrench in which protrusions are formed in the mouth opening which only engage the lateral surfaces of screw heads or nuts.